The format of colours on the web. If you are telling a browser to make a background white, for example, you write '#ffffff'. You could say 'white', but some browsers wouldn't know what you're talking about. Hex codes are 3 groups of two numbers, which in turn signify the amount of Red, Green and Blue in the colour, hence RGB. These are the three primary colours of light. The range is 0 to 9 plus A to F. So, 0 means none (therefore '#000000' is black; while F means full, resulting in white). With this knowledge, you can now create tonnes of colours by changing numbers. Although, some look better than others at all colour depths - these are the 216 safe colours.